The present invention relates generally to medical devices and methods. More specifically, the present invention relates to ultrasound catheter devices and methods for treating occlusive intravascular lesions.
Catheters employing various types of ultrasound transmitting members have been successfully used to ablate or otherwise disrupt obstructions in blood vessels. Specifically, ablation of atherosclerotic plaque or thromboembolic obstructions from peripheral blood vessels such as the femoral arteries has been particularly successful. Various ultrasonic catheter devices have been developed for use in ablating or otherwise removing obstructive material from blood vessels. For example, U.S. Pat. Nos. 5,267,954 and 5,380,274, issued to an inventor of the present invention and hereby incorporated by reference, describe ultrasound catheter devices for removing occlusions. Other examples of ultrasonic ablation devices for removing obstructions from blood vessels include those described in U.S. Pat. No. 3,433,226 (Boyd), U.S. Pat. No. 3,823,717 (Pohlman, et al.), U.S. Pat. No. 4,808,153 (Parisi), U.S. Pat. No. 4,936,281 (Stasz), U.S. Pat. No. 3,565,062 (Kuris), U.S. Pat. No. 4,924,863 (Sterzer), U.S. Pat. No. 4,870,953 (Don Michael, et al), and U.S. Pat. No. 4,920,954 (Alliger, et al.), as well as other patent publications WO87-05739 (Cooper), WO89-06515 (Bernstein, et al.), WO90-0130 (Sonic Needle Corp.), EP316789 (Don Michael, et al.), DE3,821,836 (Schubert) and DE2438648 (Pohlman). While many ultrasound catheters have been developed, however, improvements are still being pursued.
Typically, an ultrasonic catheter system for ablating occlusive material includes three basic components: an ultrasound generator, an ultrasound transducer, and an ultrasound catheter. The generator converts line power into a high frequency current that is delivered to the transducer. The transducer contains piezoelectric crystals which, when excited by the high frequency current, expand and contract at high frequency. These small, high-frequency expansions (relative to an axis of the transducer and the catheter) are amplified by the transducer horn into vibrational energy. The vibrations are then transmitted from the transducer through the ultrasound catheter via an ultrasound transmission member (or wire) running longitudinally through the catheter. The transmission member transmits the vibrational energy to the distal end of the catheter where the energy is used to ablate or otherwise disrupt a vascular obstruction.
To effectively reach various sites for treatment of intravascular occlusions, ultrasound catheters of the type described above typically have lengths of about 150 cm or longer. To permit the advancement of such ultrasound catheters through small and/or tortuous blood vessels such as the aortic arch, coronary vessels, and peripheral vasculature of the lower extremities, the catheters (and their respective ultrasound transmission wires) must typically be sufficiently small and flexible. Also, due to attenuation of ultrasound energy along the long, thin, ultrasound transmission wire, a sufficient amount of vibrational energy must be applied at the proximal end of the wire to provide a desired amount of energy at the distal end.
One continuing challenge in developing ultrasound catheters for treating vascular occlusions is to provide adequate vibrational energy at the distal end of a catheter device without overheating the ultrasound transmission wire. Generally, increasing the amount of power input to the ultrasound transmission wire causes the temperature of the wire to increase. Overheating may occur anywhere along the length of the transmission wire, from its proximal connection with the ultrasound transducer to the distal tip of the wire. Overheating of the wire, along with the mechanical stresses placed on the wire from propagating ultrasound waves, can cause wire breakage, thus shortening the useful life of the catheter device. Furthermore, it is generally desirable to ablate an occlusion via the ultrasound vibrations and not by heating the occlusion, since heating causes a denaturalization process that reduces the efficacy of the ultrasound ablation.
Some ultrasound catheters use irrigation fluid to attempt to control the temperature of the ultrasound transmission wire, but such irrigation cooling techniques are not always effective. Other devices use swapped frequencies to change frequency nodes and anti-nodes, thus moving a heat source from point to point along the transmission wire. However, a given ultrasound transmission wire resonates at the fundamental frequency for which it is designed, and thus changing frequencies essentially requires turning the ultrasound device on and off, which reduces the efficacy of the device. Some ultrasound catheter devices include one or more absorption members at the proximal end for absorbing unwanted vibrations of the ultrasound transmission wire. Such absorbers, however, do not address the heat generation issue and, in fact, may cause increased heating from frictional forces.
Therefore, a need exists for improved ultrasound catheter devices and methods that provide ablation or disruption of vascular occlusions. Ideally, such ultrasound catheters would provide a desired level of power at a distal end of the device while also preventing overheating of the device's ultrasound transmission member. Ideally, such devices would address ultrasound transmission wire overheating at its proximal connection with a catheter device as well as along the length of the wire. At least some of these objectives will be met by the present invention.